Lonewulf:
I believe it's a mental aspect vs physical appeal.
A male bottom, is basically like a woman pegging a man. She doesn't gain any tactile sensations, it's strictly a mental appreciation. It's seeing a man in a role reversal. I'd even say it's a schadenfreude situation.
No.
There are women who legitimately like to spank guys, for the same reasons that people of both sexes like to spank women. There aren't a ton of them, but they exist. In the younger generations they're more common, but I've met a few who were much older than myself as well.
Some of them like a guy's butt to be muscley and compact dome-shaped. Others like more feminine-looking guys with softer, wider bottoms. Then there are some for whom its purely psychological like you said, but there are male spankers who are also like that.